how to give size of vector in c++ code example

Example 1: c++ vector size

#include <vector>

int main() {
  std::vector<int> myVector = { 666, 1337, 420 };
  
  size_t size = myVector.size(); // 3
  
  myVector.push_back(399); // Add 399 to the end of the vector
  
  size = myVector.size(); // 4
}

Example 2: vectors c++ set the size

#include <iostream>
#include <vector>

using namespace std;

int main(void) {
   vector<int> v;

   cout << "Initial vector size = " << v.size() << endl;

   v.resize(5, 10);
   cout << "Vector size after resize = " << v.size() << endl;

   cout << "Vector contains following elements" << endl;
   for (int i = 0; i < v.size(); ++i)
      cout << v[i] << endl;

   return 0;
}